1. Field of the Invention
This invention relates to display devices utilizing an electrochemical coloration and bleaching phenomenon, that is, an electrochromic (hereinafter referred to as EC) phenomenon, and more particularly, solid state complementary EC display devices employing a reduction coloration type EC material layer and an oxidation coloration type EC material layer.
2. Description of the Prior Art
The complementary EC display device has a structure in which an EC material layer such as tungsten trioxide (WO.sub.3) which is colored in a reduction state with the application of a negative voltage and another EC material layer such as iridium oxide (Ir(OH)n) or rhodium oxide (Rh(OH)n) which is colored in an oxidation state with the application of a positive voltage are combined in a stack and both EC material layers exhibit coloration or bleaching simultaneously in the same pattern, depending on the application of the external driving voltage. When the display device is viewed, the colored state of both EC material layers can be seen through the stack.
FIG. 1 is a sectional view illustrating a conventional structure of such a solid state complementary EC display device. Referring to FIG. 1, the structure of the conventional solid state complementary EC display device will be discussed: On a transparent substrate 1, a first electrode 2 which is a thin film of a transparent conductive material such as indium-tin oxide (hereinafter referred to as ITO) is formed, and on the surface, a reduction coloration type EC material layer 3 is formed by a process of physical vapor deposition such as evaporation. As the reduction coloration type EC material layer 3, simple substances such as tungsten trioxide (WO.sub.3), molybdenum trioxide (MoO.sub.3), niobium pentoxide (Nb.sub.2 O.sub.5) and vanadium pentoxide (V.sub.2 O.sub.5) or their compounds are used.
Then, on the reduction coloration type EC material layer 3, chromium oxide (Cr.sub.2 O.sub.3) layer is formed as an ion permeable insulating layer 4 by evaporation and the like, and thereon an oxidation coloration type EC material layer 5 is formed also by evaporation and the like. As the oxidation coloration type EC material layer 5, iridium oxide or rhodium oxide is used.
Finally, on the oxidation coloration type EC material layer 5, a second electrode 6 which is a thin film of a transparent conductive material such as ITO is formed by evaporation and the like, thus making a solid state complementary EC display device.
While the coloration process of the EC display device is given several interpretations, it is generally understood as follows:
The reduction coloration type EC material is colored by the injection of cations into the EC material with the application of an electric field. On the other hand, the oxidation coloration type EC material is colored by the injection of anions. In this case, cations are protons (H.sup.+) and anions are hydroxyl ions (OH.sup.-), both of which are supplied by the Cr.sub.2 O.sub.3 layer 4 including H.sub.2 O.
In the solid state complementary EC display device thus constructed, by applying a negative voltage to the first electrode 2 and a positive voltage to the second electrode 6, the reduction coloration type EC material layer 3 and the oxidation coloration type EC material layer 5 can be colored at the same time, and by reversing the applied voltages, they can be bleached at the same time. However, conventional solid state complementary display devices have not been put into practical use because of the following defects: Since the absolute amount of H.sub.2 O contained in the chromium oxide (Cr.sub.2 O.sub.3) layer used as the ion permeable layer is limited and supplying of cations and anions to the EC material layers is insufficient, the coloration concentration useful for the display device cannot be obtained on a low driving voltage. In order to obtain the useful coloration concentration, it is necessary to increase the driving voltage, which causes not only the reversibility of coloration and bleaching of the EC materials to be impaired but the device life to be shortened. Moreover, the use of chromium oxide (Cr.sub.2 O.sub.3) as the ion permeable insulating layer causes an electric current which does not directly contribute to the coloration to flow when a driving voltage is applied across the first and the second electrodes, thus tending to increase power consumption.